The present invention relates to an air-conditioning device and to a method for operating an air-conditioning device.
Air-conditioning devices which form the basis of the generic type are known in vehicles. The purpose of air-conditioning devices is to generate and maintain a temperature stratification which is considered favorable in the interior of the vehicle and at the same time to allow the vehicle to be driven safely with respect to traffic. The air stratification that is to be generated is to be selected in such a way in terms of the temperature profile that the region surrounding the heads of the vehicle occupants is at lower temperatures than the space surrounding their feet. On the other hand, to prevent fogging in particular of the front windshield for the driver, it is desirable for the inner side of the windshield to be exposed to an air stream which is warmer in relative terms, since this air stream heats the side of the windshield on the inside of the vehicle and at the same time is able to take up a greater level of moisture. As a result, the precipitation of moisture on the inside of the windshield is prevented or very quickly cleared. Tendency to fogging may in this context occur in particular in the control region of the air-conditioning system, i.e. in the event of only a minor deviation from the control stipulations.
To achieve temperature stratification of this nature and at the same time to achieve a warm air stream in the region of the front windshield, it is known to provide a bypass passage, which branches off downstream of the heat exchanger and passes an air stream which has been heated in this way into the defrosting passage, bypassing a mixing chamber which mixes the warm partial air stream with the cold partial air stream, to be provided in an air-conditioning device. The defrosting passage is used to feed an air stream into the defrosting nozzles assigned to a vehicle windshield. The mixing chamber per se is configured in such a way that although the two partial air streams are mixed in it, there are regions in which there is a higher proportion of the cold air stream, while in other regions the proportion of the warm air stream is higher. Nevertheless, different mixed air temperatures in the air exit passages are possible depending on the region in which the air exit passages which ventilate the vehicle interior branch off, allowing the generation of temperature stratification in the vehicle interior. Air exit passages which open out further up within the vehicle in this case have lower mixed air temperatures than those which open out in the foot well.
Air-conditioning devices of this type have the drawback that a constant warm air stream is passed continuously via the bypass passage to the defrosting nozzles; this warm air stream has an air fraction which has been branched off directly downstream of the heat exchanger and is therefore at a high temperature. This air stream which is guided upward along the vehicle window, however, counteracts a temperature stratification that is to be generated in the vehicle and provides for a lower air temperature in the region of the heads of the vehicle occupants.
This drawback manifests itself in particular if the air flow which is passed through the defrosting nozzles is low, i.e. the defrosting function is switched off, and there is also no need for a large volumetric flow to generate the desired temperature stratification in the interior of the vehicle. In this case, the proportion of the warm air stream in terms of the total air stream in the defrosting passage is high and substantially determines the temperature of this total air stream. A warm air stream then emerges from the defrosting nozzles at a temperature higher than necessary to reliably generate fog-free windows, and this counteracts cooling of the interior of the vehicle and disrupts the desired temperature stratification in the vehicle interior.